Technical Field
Devices for digital fabrication of three-dimensional objects by either selectively depositing or binding raw materials together in layers.
Description of Related Art
This disclosure relates to the modification of a process and apparatus for more efficiently manufacturing three-dimensional (3D) objects using digital fabrication methods. The 3D manufacturing process, also known as additive manufacturing, rapid prototyping, or solid free form, uses digital files that describe cross sections for building the desired part and support structure. Various 3D manufacturing methods have been proposed for digitally fabricating a uniquely shaped object on a single build platform. The build rate of digitally produced 3D objects is inherently slow since 1) each 2D cross section is typically formed by a two-dimensional scanning device and 2) many 2D layers (up to thousands in a high resolution part) are required to produce an object. Furthermore, some manufacturing methods require additional time to post-process a layer before deposition of the next layer. Regardless of the method for building the 3D object, there is a general need to implement process improvements for reducing the time to build multiple, uniquely shaped 3D objects.
Various additive manufacturing systems have been proposed to produce three-dimensional objects by either selectively depositing, binding or polymerizing raw materials together in layers. The various methods include fused filament extrusion, ink jetting, selective laser sintering, powder/binder jetting, electron beam melting, stereolithography and electrophotography processes. In general, the various methods tend to exhibit a slow build rate. For example, many of the selective deposition methods have a slow build rate since the deposition of the build and support materials is generally provided by a scanning head for extruding or jetting the material for each layer. To improve the build rate with a fused filament extrusion method, a 3D printer from Cartesio called the CartesioLDMP has multiple extruder heads for simultaneously printing multiple similar shaped 3D objects on a single stationary platform.
The 3D manufacturing method based on electrophotography has the potential of improving the build rate since it is well known from the electrophotographic industry that 2D layers of imaged powder can be formed and deposited in a time less than about half of a second. However, the 3D object build rate utilizing the electrophotographic method is decreased if a wait time required before depositing another layer is comparable to or greater than the build time for each layer.
The electrophotographic process can enable high 2D layer formation rates since the imaging of a uniformly charged photoreceptor is provided by light exposure from either a scanning laser beam or LED imaging bar. The deposition of powder material is provided by a high process speed powder development system. Typically, insulative powder is triboelectrically charged in a development system. Electrostatic forces acting on the charged powder are used to develop an electrostatic image formed by laser or LED light exposure of a uniformly charged photoreceptor. Likewise, electrostatic forces are used to transfer the charged powder image on the photoreceptor to a medium such as paper or an intermediate roll or belt. The electric field for electrostatic transfer of the charged powder to the medium is typically provided by either depositing gaseous ions on the backside of the medium or applying an electrical bias to a stationary or roller electrode behind a belt or roll comprised of a charge-relaxable material or overcoating, respectively. In electrostatically transferring charged powder to a medium such as paper or an intermediate roll or belt, one can apply rather high electric fields (40 volts per micrometer) for efficient powder transfer without air breakdown limitations. The high transfer efficiency is not degraded in color electrophotographic printers that require the electrostatic transfer of several charged powder layers involving different combinations of cyan, magenta, yellow and black toner layers.
Although it is recognized that the electrophotographic process can enable rapid formation of 2D layers, a number of researchers have reported problems in producing arbitrarily thick 3D objects when using conventional electrophotography to produce charged powder depositions that are repeatedly electrostatically transferred and heat fused to the object being built. For example, a publication on “Transfer Methods toward Additive Manufacturing by Electrophotography” by Jones et al. in the conference proceedings of the Society for Imaging Science and Technology (IS&T), NIP27: 2011 International Conference on Digital Printing Technologies, pp. 180-184 reports the use of a conventional monochrome printer with conventional toner for repeatedly electrostatically transferring a uniformly deposited toner layer onto a moving platform and heat fusing the layer before the next deposition cycle. It was noted that after about 20 transfers, the surface had many defects and irregularities that compromise the quality of the object. It was remarked that every research group, to the author's knowledge, encountered the same type of surface defects when attempting to deposit non-conductive toner with stack heights in excess of 1 mm.
In spite of the surface defects problem encountered after many electrostatic transfers of charged powder to build an object, the effectiveness of the conventional electrostatic transfer process diminishes as thickness of the object increases. When the electrostatic force for transferring charged powder is provided by an electric field due to an electrical bias between the conducting substrate of the build object and the ground plane of the photoconductor, the applied electric field and correspondingly the electrostatic force decreases with increasing thickness of the object. Furthermore, the accumulation of charge on the object due to the charge on the transferred powder creates an electric field that suppresses powder transfer and therefore limits the thickness of the build object and causes irregularities in the surface. To suppress the build limitation due to charge accumulation on the object, the feasibility of using a corona (gaseous ions) charging device for charging the top layer of the object with a polarity opposite to that of the toner has been reported in a publication by A. Dutta on “Study and Enhancement of Electrophotographic Solid Freeform Fabrication” as a Masters of Science thesis in the Department of Mechanical and Aeronautical Engineering at University of Florida, Gainesville, Fla. in 2002. (See http://etd.fcla.edu/UF/UFE0000527/dutta_a.pdf.) The top charging method doubled the thickness of the build object from 1 mm to 2 mm before surface quality degradation was observed. Although objects thicker than 2 mm could be produced, the surface defects became exaggerated with each successive transfer. A publication by A. Kumar Das on “An Investigation on the Printing of Metal and Polymer Powders Using Electrophotographic Solid Freeform Fabrication” as a Masters of Science thesis in the Department of Mechanical and Aeronautical Engineering at University of Florida, Gainesville, Fla. in 2004 (See http://etd.fcla.edu/UF/UFE0005385/das_a.pdf) suggests that although the corona charging counteracted the powder charge in the initial layers, its effectiveness was diminished for a thicker object.
To circumvent the 3D object build thickness and surface irregularity problems associated with electric field transfer of charged powder from electrophotographic images to the build object, an alternative approach of using of heat and pressure to transfer the charged powder layers to a build object has been described by a number of researchers. The first to disclose the utilization of heat and pressure to build 3D objects from electrophotographic produced powder layers was Bynum in U.S. Pat. No. 5,088,047 (1992). This patent discloses the use of an electrophotographic print engine to deposit layers of toner on a TEFLON® (polytetrafluoroethylene) coated belt. Each layer on the belt was made tacky by heating or exposure to solvent vapor before being transferred to the build object with a combination of heat and pressure. Other relevant patents include U.S. Pat. No. 5,593,531 issued to Penn, U.S. Pat. No. 6,066,285 issued to Kumar, U.S. Pat. No. 6,780,368 issued to Liu and Jang, and U.S. Pat. No. 8,488,994 issued to Hanson et al. In these U.S. patents, a transfer medium is configured to receive and transfer imaged layers of a thermoplastic-based powder from an electrophotographic imaging engine. Before the imaged layer is transferred to the build object or support material, a heater is used to heat the imaged layers on the transfer medium to at least a fusing temperature of the thermoplastic-based powder. The system also includes a layer transfusion assembly comprising a build platform where the layer transfusion assembly is configured to transfuse the heated layers in a layer-by-layer manner onto the build platform to print the 3D object. The system usually also includes a post-transfusing cooling unit configured to actively cool the transfused layers to maintain the printed 3D object at about an average temperature that is below a deformation temperature for the 3D object. The utilization of heating and cooling cycles of the transfer medium and 3D part/support materials in the transfusion process builds in a wait time that limits the overall speed of the electrophotographic method for digitally producing 3D objects. The disclosures of these U.S. Pat. No. 5,088,047 of Bynum; U.S. Pat. No. 5,593,531 of Penn; U.S. Pat. No. 6,066,285 of Kumar; U.S. Pat. No. 6,780,368 of Liu et al.; and U.S. Pat. No. 8,488,994 of Hanson et al. are incorporated herein by reference.
Although there is no evidence from the Bynum patent that this disclosure was reduced to practice, the publication by Jones et al. on “Transfer Methods toward Additive Manufacturing by Electrophotography” in the conference proceedings of the Society for Imaging Science and Technology (IS&T), NIP27: 2011 International Conference on Digital Printing Technologies, pp. 180-184 indicates that other researchers have subsequently developed hardware and published experimental results using some combination of heat and pressure for transfer and fusing electrophotographic produced powder layers to produce 3D objects. For example, the Jones et al. 2011 publication describes the use of an industrial laser (electrophotographic) printer and infrared heaters to assess the maximum thickness one can achieve in building a 3D object according to the Bynum transfer approach. It was learned that stack heights are limited to about 1 mm to 2 mm before quality issues due to surface irregularities prevent further build thicknesses.
The fact that the quality of the 3D objects produced by the heat and pressure transfer method is not substantially improved over the quality of such objects produced by the electrostatic transfer method (including an electrostatic conditioning of the object during the building) has been discussed by Jones et al. in the conference proceedings of the Society for Imaging Science and Technology (IS&T), NIP28: 2012 International Conference on Digital Printing Technologies, pp. 327-331. The implication is that during the build of the 3D object, charge accumulation on the object due to the charge of the transferred powder and possible contact charging by the heated transfer roller is the cause of non-uniform transfer and consequently unacceptable 3D object quality when the build thickness is typically greater than about 1 to 2 mm. It was suggested that acceptable 3D object quality produced by the electrophotographic method is reliant on managing the residual charge on the build object.
From a review of the literature and patents, it is clear that the build rate of digitally produced 3D objects based on the electrophotographic process is limited by a wait time associated with the transfer of each 2D layer to the build object. Heat is typically employed to render the 2D layer sufficiently tacky during a transfusion step that also uses pressure to adhere the layer to the build object. Furthermore, the build object can be either charged or neutralized with gaseous ions to improvement the quality of the build object. Whenever heating is used such as in the transfusion step, there is a wait time introduced in the process that depends on heating rates, thermal conductivities, heat capacities, and cooling rates. The productivity for digitally building a 3D object on a build platform with the electrophotographic process is compromised if the thermal wait time for applying another layer exceeds the time that it takes to produce a 2D layer by the electrophotographic process.
In summary, in the patents, published applications, and literature known to the Applicants that describe various methods including electrophotographic methods, for digitally fabricating 3D objects, such methods are limited to a single build platform architecture. If the lateral size of the 3D object is smaller than the build platform size, it is possible to produce multiple 3D objects on a single platform. However, the productivity for producing 3D objects is not substantially improved for 3D fabrication methods in which the layer formation is based on a single 2D scanning system. After a 3D object is fabricated on any single build platform using a particular process, the 3D object must either be removed from the build platform, or the build platform with the 3D object must be removed and replaced with another build platform for fabricating another 3D object of the same or different shape. Under such constraints, the rate for producing multiple 3D objects is undesirably slow when using an apparatus and process that is limited to a single build platform.
Accordingly, there remains a need for a high build rate method and apparatus, which can build a three-dimensional part.